Looks no answers to this problem. Facing same issue but not sure is it related with Garmin Connect or some Chrome feature.
Have tried to switch on and off different things on Chrome but still no sucess. So if someone would have some great idea what to try - beside another browser - then I would like to hear.
Same problem on Chrome and Firefox. This happens about 3-4 days a week. Of course it only happens on my computer, not my phone or Chromebook, so I can see my calendar but not send scheduled workouts to my watch because only my computer has the software to do that and I can't see the calendar there! So frustrating.
UPDATE: I contacted support, and their response was "After the update I've seen a number of customers who have had to clear their browser cache and temporary files to resolve website issues. You can reference this article how to do this on various browsers. Clear the browser history, cache, and temporary internet files."
I'm not sure which update he was referring to though. Was it the switch to Modern (because that's when the site got all buggy for me), the new layout, or some other more recent update that made things even worse? Anyway, I cleared the cache on my browsers and it seems to be working now.